The present invention relates to a method for controlling the material feed to a roller press for grinding particulate material, by which method the following stages are taken in succession:
1) a first value indicating the operating mode of the roller press is measured during the roller press operation
2) the material .feed rate to the roller press is forcibly changed
3) the new value indicating the operating mode of the roller press is measured during the roller press operation
4) the calculated differential value is compared with a first prefixed numerical value and a second prefixed numerical value, the second prefixed numerical value being greater than the first prefixed numerical value, and the material feed rate to the roller press as a function hereof is either reduced, increased or maintained unchanged.
5) the material feed rate to the roller press as a function of the calculated differential value is either reduced, increased or maintained unchanged.
A roller press of the above kind and its function are known from, for example, U.S. Pat. No. 4,357,287.
In principle, material may be fed to the roller press in two ways, viz. by shaft feeding or by so-called starve feeding.
In the case of shaft feeding, the entire roller gap and an area thereabove is filled with material to be ground, enabling the rollers to draw a substantially constant amount of material into the gap between the rollers. Since, simultaneously, the grinding bed thickness/gap width as well as the nip angle are at the maximum attainable levels, and since the feed shaft is never run empty of material, this means that the roller press can be operated at optimum capacity.
However, shaft feeding has the disadvantage, particularly when finish-grinding fine-grained products, that fluidization problems may occur with respect to the material in the feed shaft, hence resulting in irregular and unacceptable roller press operation. Fluidization may, for example, occur as a result of the material in the feed shaft being scavenged by air expelled from the compaction zone of the roller press. The tendency towards fluidization may be reduced to a certain extent by decreasing the roller speed, but this will cause the roller capacity to be correspondingly reduced.
This problem is remedied by means of starve feeding i.e. by reducing the material feed to the roller press to such an extent that the roller gap is not entirely filled with material, thereby avoiding a material column over the rollers. At the same time, this makes it possible to apply a higher roller speed, whereby a higher roller press capacity is obtained.
In the case of starve feeding the material feed rate is normally controlled in a manner ensuring that, during shortage feeding of the roller press, the grinding gap is maintained at a level which is slightly lower than the maximum gap width, so that the feed is reduced in steps of varying magnitude in case of a substantial build-up of material in the feed shaft. However, the roller press is not utilized to full capacity when performing this type of control, since, for reasons of safety, the selected grinding bed thicknesses applied during starve feeding are much lower than the maximum values.
The capacity of the roller press may be improved by controlling the material feed to the roller press as a function of the deviation from a predetermined size of an operating value such as, for example, the gap width or the power consumption. According to this method, a trial run is conducted with the material to be ground in order to establish the maximum size of the operating value according to which the material feed is to be controlled, e.g. the gap width. During operation of the roller press the material feed is subsequently controlled so that this value (e.g. the gap width) is kept at a lower level (10-15%) than the maximum value, being maintained at a substantially constant level, e.g. by means of a PID regulator.
However, this method has the disadvantage that it is an underlying assumption that the material to be ground is reasonably homogeneous, due to the fact that, for example, the gap width and the necessary grinding effect will vary considerably according to variations in the particle distribution, density, porosity or moisture of the material. So, even though the mentioned method may improve the control of the material feed, and hence the roller press operation, it will be difficult to achieve optimum roller press capacity at the same time as the shaft feed problems are avoided.
From the U.S. Pat. No. 4,611,763 a method is known for controlling the material feed to a tube mill for grinding of particulate material, by which method a controller is utilized which controls the material feed to the mill by comparing an initial setpoint input with a signal generated by various monitoring devices. According to the patent specification the output signal of the controller is integrated for a predetermined time whereupon the setpoint is automatically increased a predetermined amount and the new output signal of the controller is integrated for the same predetermined time. The original and the new integrated value are then compared, and if the new value is greater than or equal to the original integrated output value, the setpoint is again automatically increased the predetermined amount and the next new output signal of the controller is integrated for the same predetermined time. The next new value is then compared to the immediately preceding value and if the latter is greater than or equal to the preceding value, the setpoint is again increased automatically. If the last output value obtained from the controller is less than the immediately preceding output value, the setpoint will be decreased the predetermined amount. By this method the setpoint is hence increased or decreased so that the same predetermined amount, depending upon whether the last obtained output value from the controller, is greater than, equal to or less than the immediately preceding value.
If the aforementioned method were to be used for controlling the material feed for a roller press, this would involve a certain risk of the feed shaft being overfilled, since, according to the method, the feed rate will always be increased if two successively performed measurements produce the same result, as would be the case for a roller press operating on shaft feeding.
According to the known method the control operation is further based on the performance of simultaneous measurements of several different operating parameters for the mill, which, on a combined basis, form the basis for the calculation of the efficiency value prior to and subsequent to an effected change in the material feed rate, which prior and subsequent values again form the basis for calculating the value which determines whether the material feed should be increased or decreased. Consequently, the control operation appears to be rather complicated and, further, it is based on a theoretically defined efficiency parameter.
Further, the known method does not allow intervals to be applied between the control sequences since the control sequences overlap one another in that the output value by one control sequence indicates the termination time of one period, whereas the same value by the subsequent control sequence indicates the starting time for the new period.